Generally, a conventional automatic transmission for vehicles includes a torque converter, a multi-stage gear shift mechanism connected to the torque converter, and a plurality of friction elements actuated by hydraulic pressure for selecting a gear stage of the gear shift mechanism.
In such hydraulic control systems, hydraulic pressure generated by a hydraulic pump is supplied to friction elements and a plurality of control valves such that automatic shifting is realized in accordance with a driving state of the vehicle and engine throttle opening.
The above hydraulic control system generally comprises a pressure regulating controller for controlling hydraulic pressure generated by the hydraulic pump, a manual and automatic shift controller for selecting a shift mode, a hydraulic pressure controller for controlling shift response and shift quality for smoothly selecting a shift mode during shifting, a damper clutch controller for actuating a damper clutch of the torque converter, and a hydraulic pressure distributor which supplies an appropriate amount of hydraulic pressure to each of the friction elements.
The hydraulic pressure distributor distributes a variable amount of pressure in accordance with ON/OFF or duty control of solenoid valves by a transmission control unit such that shift control is realized.
When changing shift stages, the timing of exhausting hydraulic pressure from one set of friction elements and providing hydraulic pressure to another set of friction elements greatly influences shift quality. Further, when mis-timing occurs, an abrupt increase in engine revolutions or locking of the shift mechanism can result.
In order to improve shift quality by accurately controlling the timing of pressure supply, a method of modifying shift valve structure has been developed. However, such a method complicates both the structure of the shift valves and the control process.